Zone 6, You probably should have divided a week or two ago. Still, don't disturb too much and they should be fine. Better than June.

flowerfan is correct - PC iris are a bit fussy about root disturbance at the incorrect time of year. Fall is really ideal for dividing or transplanting of these. In general, fall division is recommended for spring/early summer blooming perennials and early spring for later blooming perennials. Having said that, I've divided Sibes at all different times of the year with good success, however they often miss a season of bloom if I did it at any time other than early fall.

what they said.. but lets simplify it a bit ... the plant itself doesnt matter.. there need not be different rules for different plants .. in this regard ... the simple question is.. do you want flowers this year ... if you do ... you sharpen your shovel.. and cut away half.. the half left.. will probably bloom ... with no insult to the root mass left ... if you simply plot the other half in a new spot.. and treat it well.. it will also probably bloom .. but might not ... if you divide the second half.. into a bazillion little pieces ... they might bloom next year ... never forget.. flowering is sexual maturity ... and a babe has to grow a mature root mass ... to flower above ... so the smaller you go on dividing.. the longer it will take to clump and flower ... you need the simple rule .. because.. often .. you have to do.. what you have to do.. when you have the time ... too many projects get left behind.. and never done.. because you think.. oh.. i will come back and do that in fall .. and then next spring.. you are standing there.. back at square one ... lol ken

Did you follow these guidelines: Two to four fan divisions are recommended for transplanting, and the roots must be kept moist while the plants are out of the ground. Plant the rhizomes one inch deep (slightly deeper in sandy soils). The information came from http://www.socsib.org/ssicult.htm and according it you are later than recommended. Keep them well watered and fingers crossed.